A conventional derailleur mechanism for a bicycle, as disclosed in Taiwanese Patent No. I391294 or No. I336304, includes a plurality of front sprockets adapted to be securely disposed on a crank axle of the bicycle, a plurality of rear sprockets adapted to be securely mounted to a rear hub of the bicycle, a chain trained on one of the front sprockets and one of the rear sprockets, and front and rear derailleurs respectively disposed at positions corresponding to the front and rear sprockets for derailing the chain onto different ones of the front and rear sprockets.
The conventional derailleur mechanism is applied widely to bicycles, but operation is limited by the number of the front and rear sprockets available. Therefore, there is room for improving the conventional derailleur mechanism since stepless control of the speed of the bicycle may be desirable.